Electrical stimulation therapy may be used for various forms of treatment. For example, stimulation therapy may be provided to address neurological issues such as chronic pain, tremors, and the like. In such an example, an implantable stimulation device is typically located in one location of convenience and is connected to electrical leads that are routed to a stimulation site such as within the brain, within the spinal column, within the pelvic region, or elsewhere. The electrical leads include electrodes that interface with the tissue at the stimulation site to deliver the stimulation signals from the stimulation device.
It may be useful to also sense physiological signals nearby the stimulation site. Such signals may be useful to tailor the stimulation therapy to the particular condition being treated and/or to better understand the response of the tissue nearby the stimulation site to the stimulation signals. Sensing physiological signals may be especially useful where the stimulation therapy may be at least partially controlled based on the physiological signals.
Sensing physiological signals in proximity to the stimulation site while the stimulation signals are being applied is problematic. The sensing electrodes receive the physiological signals with amplitudes in the microvolt range while also receiving the stimulation signals that have amplitudes reaching tens of volts. Thus, the physiological signal is too small relative to the stimulation signal to be detected. Additionally, the relatively large amplitudes of the stimulation pulse and recharge pulse of the stimulation signal can produce lingering voltages within the sensing circuits that overly saturate the sensing circuits and cause the physiological signals to be obscured even during periods when the stimulation pulse and the recharge pulse are not present.
This is particularly troubling for neurological stimulation systems. In neurological stimulation, the recharge phase occupies the majority of the time between stimulation pulses. For instance, a neurological stimulation signal may have a period of 7 milliseconds while the recharge phase occupies 5 of the 7 milliseconds. Thus, it is not feasible to simply blank out the sensing of the stimulation pulse and the recharge phase because there is an inadequate amount of time remaining within the period to sense the physiological signal.